In general, it is often desirable to change the natural duration of laser pulses produced by pulsed lasers. More specifically, the development of devices and apparatus which generate compressed or shortened laser pulses has been found to be advantageous for a plethora of applications. To this end, three basic mechanisms have been developed to produce compressed or shortened laser pulses. These basic mechanisms are mode-locking, Q-switching, and cavity-dumping. Traditionally, these three mechanisms have been employed as part of a separate low-power pulse generator. The laser output from the pulse generator is then fed into a regenerative amplifier where the energy of the individual pulses is amplified to become the overall laser output.
Practice has demonstrated, however, that the employment of lasers which include a pulse generator that is separate from the regenerative amplifier is subject to certain disadvantages. For example, it is often the case that the size and complexity of such systems has limited their applicability in real world situations. As a result, laser systems which include the basic pulse compression mechanisms within the laser cavity of the regenerative amplifier have received increasing attention. A laser of this type is described in an AT&T memorandum dated Sep. 16, 1986, authored by Dulling et al., in which it was suggested that the basic mechanisms of mode-locking, Q-switching, and cavity-dumping all be combined within the laser cavity of a regenerative amplifier.
So-called "all-in-one" pulsed laser beam generators, of the type proposed by Dulling et al., employ a time-sharing methodology where the single laser cavity is first used to generate a sequence of short-duration laser pulses. Once the pulse-shaping or pre-lasing period is complete, the laser cavity is used to regeneratively amplify the pulse train which becomes the laser output.
Inclusion of the basic mechanisms of mode-locking, Q-switching, and cavity-dumping within the laser cavity of a regenerative amplifier allows all-in-one laser systems to offer a compact layout which can be highly advantageous in real-world situations. It may be appreciated, however, that traditional methods for implementing these basic methods within the laser cavity may result in a laser system with an unacceptably high cost. More specifically, it may be appreciated that the functions of Q-switching and cavity dumping have traditionally been performed using separate active elements, each of which must be controlled and coordinated during the duty cycle of the laser system. This approach complicates the design of the all-in-one laser and may result in excessive system cost. For this reason, it is desirable to produce an all-in-one laser which combines the functions of Q-switching and cavity dumping within a single active element.
Unfortunately, all-in-one pulsed laser systems have certain operational characteristics which, if they are not properly controlled, can limit their efficacy. Specifically, substantial leakage of low intensity laser pulses can occur during the generation of a pulsed laser beam if the various mechanisms of the system are not effectively coordinated and employed. The primary cause of this leakage is due to system limitations which result from the requirement that, in order for mode-locking to occur, it is necessary that the laser medium be pumped to emit light. As a result, there is a tendency for the regenerative amplifier to prematurely amplify the pulse train before the pulse-shaping period has completed. In order to prevent a runaway amplification of the pulse train before completion of pulse-shaping, the tack has been to eject part of the pulse train from the laser cavity using a slightly misaligned quarter-wave plate (.lambda./4) on the cavity laser beam path. The result is that an uninterrupted train of pulses are emitted from the cavity. While some of this leakage may be acceptable, extensive leakage of laser pulses from the cavity, even though they be of low intensity, can have adverse effects in many applications. This is particularly so when the pulsed laser system is to be used for ophthalmic surgery.
The present invention recognizes that unwanted leakage of laser pulses from the laser cavity can be minimized by properly controlling the pumping of the laser medium. Specifically, rather than relying on misaligned quarter wave plates to prevent runaway amplification, the present invention recognizes that by pumping the laser medium in a quasi-continuous fashion during the period of pulse-shaping, runaway amplification may be avoided.
In light of the above, it is an object of the present invention to provide a pulsed laser beam generator which provides a high-frequency, high-power pulsed output. Another object of the present invention is to provide a pulsed laser beam generator which combines the basic pulse compression mechanisms and amplification means within a single laser cavity. It is still another object of the present invention to provide a pulsed laser beam generator which minimizes leakage of laser energy during the pulse-shaping process. Still another object of the present invention is to provide a pulsed laser beam generator which combines the functions of Q-switching and cavity dumping within a single active element. Yet another object of the present invention is to provide a pulsed laser beam generator which is relatively simple to use, relatively easy to manufacture, and comparatively cost effective.